Each rule consists of a specific field (or "any field"), some criteria that need to be met for that field, and possibly some sub-rules. When you put more than one rule together, you can tell MusicBee that it needs to match ANY or ALL of them in order for your conditions to be satisfied.

The second drop-down box contains operators such as "is not" or "contains", as appropriate for each tag. What operators are available for each field are determined by the field type. For custom and virtual tags, you can change the field in Configure Fields. If you're not happy with the automatically generated options, you can make these rules as complex as you need using "match RegEx" and "match RegEx/i" (the latter is case-insensitive) operators.

The third drop-down box contains a value that you define (either free text or from a list, depending on the tag), [playing track], or a regular expression. (If you need help with regular expressions, aka regex, try this page.) Items in lists (such as for matching the "is any of" or "is not any of" rules) should be separated with a semicolon, or you can click [...] and make a list in the box that pops up).

Sub-rules can be added for each rule via the [.] button. They operate exactly like other rules, except that they are applied as AND rules only for the main rule they are attached to.